Generally in the production of pulp suitable for formation into paper, wood or other raw cellulosic fibrous material, is subjected to chemical digestion in a pulping liquor to form a pulp of the cellulosic fibrous material. The pulp thereafter is subjected to brightening and purification operations in a bleach plant.
The spent pulping liquor usually is subjected to a series of recovery and regeneration operations to recover pulping chemicals and to provide fresh pulping liquor. Generally, the pulping liquor contains sodium hydroxide and other sodium salts.
A widely used pulping process is the Kraft process. While the present invention will be described hereinafter with particular reference to Kraft mills, the process is applicable also to other pulp mill operations involving the use of sodium hydroxide as pulping chemical and which utilize spent chemical recovery and pulping chemical regeneration operations. Many such operations involve the use of sulphur-containing chemicals in addition to the alkali metal hydroxide, including the Kraft process, high yield pretreatment Kraft process and other processes. Other pulping operations which may be employed include the soda process and the soda-oxygen process. The present invention also is applicable to cross-recovery operation used in mixed systems.
In the conventional Kraft process, raw cellulosic fibrous material, generally wood chips, is digested, by heating, in a pulping liquor, known as white liquor and containing sodium sulphide and sodium hydroxide as the active pulping chemicals to provide a pulp and spent pulping liquor, known as black liquor. The black liquor is separated from the pulp by washing in a brown stock washer and the pulp then is passed to the bleach plant for brightening and purification operations.
The black liquor then is passed to the recovery and regeneration system in which the black liquor first is concentrated, usually by evaporation, and the concentrated black liquor is burned in a furnace to yield a smelt containing sodium carbonate and sodium sulphide. A sodium-and sulphur-containing compound, generally sodium sulphate, is added to the black liquor generally prior to feed of the concentrated black liquor to the furnace, although such sodium-and sulphur-containing compound may be added at any other convenient point, such as to the white liquor prior to the digestion step to make up sodium and sulphur values lost from the recovery system.
The smelt is dissolved in water to yield a raw green liquor which then is clarified to remove undissolved solids. The clarified green liquor is causticized with reburned lime whereby the sodium carbonate is converted to sodium hydroxide. The resulting liquor is white liquor which then may be recycled to the digestion step to provide at least part of the pulping liquor.
Bleach plant operations generally involve a sequence of brightening and purification steps, together with washing steps. The brightening steps generally involve the use of bleaching agents. In the present invention, at least one of the brightening steps preferably involves the use of at least one chlorine-containing bleaching agent. Such chlorine-containing bleaching agents include chlorine, chlorine dioxide, chlorine monoxide and sodium hypochlorite.
The purification steps generally involve treatment with sodium hydroxide solution, and usually is known as a caustic extraction step. In some instances, the bleaching and caustic extraction steps may be combined, for example, using the so-called "oxygen bleaching" operation. Where the oxygen bleaching operation is used, however, it is used in this invention preferably in combination with one or more bleaching operations using chlorine-containing bleaching chemicals.
A particular bleaching operation which has been employed involves an initial bleaching of the pulp with an aqueous solution containing chlorine or a mixture of chlorine dioxide and chlorine, an intermediate washing, a caustic extraction using aqueous sodium hydroxide solution, a further washing, a bleaching with an aqueous solution of chlorine dioxide, another washing, a further caustic extraction using aqueous sodium hydroxide, an additional washing, a final bleaching with chlorine dioxide solution and a final washing. This is the socalled CEDED operation. The present invention will be described with particular reference to this procedure, although other procedures may be employed, such as, the use of an aqueous solution containing approximately 100% chlorine dioxide in the first bleaching step.
The above-described CEDED operation may be carried out using the so-called "Dynamic Bleaching" process claimed in Canadian Pat. No. 783,483. In this process, pulping treating solutions are passed successively through a mat of fibers in which the fibers are maintained relative stationary with respect to each other. Washing steps, except for a washing after the last step of the bleaching and purification sequence may be omitted.
The spent wash waters from bleach plants generally have been discharged to water bodies, such as streams, rivers, lakes and oceans, without any attempt to recover chemicals therefrom, although in some instances solid particle recovery operations have been made. One of the main reasons that no attempt has been made to recover these chemicals is because they are very dilute and of small value. The bleach point also produces spent bleaching liquor effluents and spent caustic extraction effluents. These effluents have objectionable colour and are toxic and harmful to aquatic and marine biota and polluting since they contain fibers and materials consuming oxygen present in the water. It is desirable to avoid such environmental pollution, and hence avoid the discharge of these effluents from the mill.
Due to the use of chlorine-containing bleaching chemicals and sodium-containing purification agents, the spent wash water contains substantial quantities of sodium chloride. In addition, when the spent bleaching liquor effluents and the spent caustic extraction liquor effluents are extraction liquor effluents are mixed at least part of the residual chlorine and soda values combine to form sodium chloride. In the present invention, the normally discharged effluents, namely, the spent wash water, the spent bleaching chemicals and the spent caustic extraction liquor preferably are mixed to provide a bleach plant effluent stream, known as BPE.
The quantity of the chlorine containing bleaching agents and the quantity of sodium hydroxide used as caustic extraction liquor preferably are balanced to provide about one atom of sodium for each atom of chlorine, whereby these chemicals form sodium chloride. In practice, the quantity of sodium hydroxide solution is in a slight stoichiometric excess to ensure the conversion of all the chlorine values to sodium chloride. The equivalence of sodium and chlorine atoms in the bleach plant effluent is preferred so that the overall sodium inventory of the mill thereby remains unchanged. In the CEDED sequence when chlorine or mixtures of chlorine dioxide and chlorine in which the proportions of available chlorine provided by chlorine dioxide is low, is used in the first stage an amount of sodium hydroxide solution in excess of that required for extraction must be added to match the chlorine atoms present. If no excess is added, only about 40 to 50% of the chlorination stage filtrate can be recovered to match the stoichiometric equivalent of sodium atoms used in the extraction. Where, however, the available chlorine is provided predominantly by chlorine dioxide, typically above about 70%, the quantities of sodium and chlorine atoms are substantially equivalent and hence it is preferred to employ the latter sequence.
Sodium chloride present in the pulp mill system may arise from other sources. Such sodium chloride may be present where the logs are floated in sea water prior to formation of wood chips therefrom. The use of sea-borne logs also causes there to be present in the black liquor sodium chloride and other alkali metal chlorides from washing of the pulp in the brown stock washer. In addition, if brackish water is used to provide process make up water, sodium chloride again is present to the bleach plant effluent.
In the present invention, the bleach plant effluent preferably is added to the spent pulping liquor recovery and regeneration operation, and in this way retains this effluent within the mill. It has been proposed previously in Canadian Pat. No. 832,347 and U.S. Pat. No. 3,698,995 to reduce the environmental problems of bleach plant effluent by utilizing the spent wash waters to wash the pulp in the brown stock washer. The use of the spent wash water in this manner reduces the overall water requirement of the mill. In the present invention, it is preferred to use the bleach plant effluent, consisting of a mixture of spent wash waters, preferably provided by countercurrent washing as described in Canadian Pat. No. 832,347 and U.S. Pat. No. 3,698,995, spent bleaching chemicals and spent caustic extraction liquor, to wash the pulp in the brown stock washer and provide thereby an "effluent-free" pulp mill.
In a particular manner of carrying out such countercurrent washing in a CEDED sequence in which the pulp is washed after every step, there is a completely countercurrent flow of liquors, namely spent bleaching liquors, spent caustic extraction liquors and wash water, with respect to the flow of pulp through the bleach plant. In such an operation, fresh water or white water is passed into contact with the pulp following the last bleaching stage, the spent wash water from this washing being mixed with spent chlorine dioxide from the last bleaching stage. The resulting mixture is split into two streams, the bulk being used to wash the pulp from the last caustic extraction stage, and the remainder being used to mix with spent caustic extraction effluent from the last caustic extraction stage, the mixture being used partially to mix with effluent from the washing step after the intermediate bleaching stage.
The remainder of the mixture is used as wash water to wash the pulp from the intermediate bleaching stage. Spent bleaching chemical from this stage is mixed with the aqueous material resulting from the last mixing, the resulting material, representing the combined effluents from the subsequent steps of the bleach plant, being used partially to wash the pulp from the first caustic extraction stage and partially to wash the pulp from the first bleaching stage. The spent wash water from the washing of the pulp from the first caustic extraction stage is mixed with spent caustic extraction liquor from the first caustic extraction and the mixture is divided into two streams, one of which represents an alkaline effluent and the other is used as wash water for the pulp from the first bleaching stage. The spent wash water from this stage is mixed with spent bleaching chemical from the first bleaching stage to provide an acid liquor, part of which constitutes an acid bleach plant effluent.
Another part of the acid liquor may be used to mix with the pulp received from the brown stock washer to provide the water required to bring the pulp to the consistency necessary in the first bleaching stage. A further part of the acid liquor is used as the aqueous medium for gaseous chlorine used in the first bleaching stage. In this way a portion of the acid liquor, constituted by the latter two parts may be recycled in the first bleaching step.
A further part of the acid liquor may be used as wash water to displace into but not through the unbleached pulp on the unbleached decker so that it is substantially all discharged to the unbleached decker chest along with the unbleached pulp.
Still another part of the acid liquor may be used as wash water to wash the screen on the first bleaching stage washer so that it mixes with spent bleaching chemical from the first bleaching stage and is recycled to become part of the acid liquor.
The acid spent bleaching chemical liquor is mixed with the alkaline caustic extraction liquor to provide the bleach plant effluent.
The bleach plant effluent may be introduced at other stages of the recovery and regeneration operations. Further, the bleach plant effluent may be split into two or more streams which are introduced at different locations of the recovery and regeneration operations, for example, to wash calcium carbonate mud to provide the "weak wash" water or to dilute concentrated white liquor. In addition, portions of the acid spent bleaching chemical liquor and/or the alkaline caustic extraction liquor may be separately introduced at different locations of the recovery and regeneration operation.
The quantity of sodium chloride present in the bleach plant effluent varies depending on the bleaching sequence which is employed. In a typical procedure where a mixture of chlorine dioxide and chlorine are utilized in the first stage of a CEDED sequence the quantity of sodium chloride may vary between about 120 and 160 lbs/ton of pulp depending on the proportion of chlorine dioxide used. Typically, when the total available chlorine in the first stage is provided 70% by chlorine dioxide and 30% by chlorine, the quantity is about 120 lbs/ton of pulp.
The introduction of the bleach plant effluent to the pulping liquor recovery and regeneration operations closes the whole system and sodium chloride is not purged by way of discarded bleach plant effluent. The sodium chloride remains unconverted by the black liquor recovery steps and hence would build up in the system. In order to prevent such build up and at the same time utilize the concepts of an effluent-free pulp mill, it is essential to remove sodium chloride from the system. Such removal of sodium chloride should be such that the other valuable components utilizable as or convertible into pulping chemicals are not removed from the system along with the sodium chloride. In addition, it is preferred to remove a quantity of sodium chloride from the mill equivalent to the amount introduced to and/or produced within the mill, typically about 120 lbs/ton pulp.
The process described and claimed in Canadian Pat. No. 915,361 and the corresponding U.S. Pat. No. 3,746,612 makes it feasible to provide an effluent-free pulp mill by removing from the mill on a continuous basis an amount of sodium chloride equivalent to the amount of chloride introduced to the recovery and regeneration system. This is achieved by concentrating, preferably by evaporation, the white liquor prior to recycle to the digestion stage in order to precipitate and remove sodium chloride from the white liquor.
In the causticization of the sodium carbonate to sodium hydroxide, less than 100% efficiency is experienced, and upon concentration of the white liquor to precipitate sodium chloride, the unconverted sodium carbonate, i.e. an unregenerated pulping chemical, also is precipitated. As described in Canadian Pat. No. 915,362 and its corresponding U.S. Pat. No. 3,740,307, pure sodium chloride is recovered from the material precipitated in the white liquor concentration step and an aqueous solution of the sodium carbonate precipitate together with some of the sodium chloride is formed and this aqueous solution is recycled to the green liquor, so that the sodium carbonate values are not lost from the mill.
It now has been found that in many instances the spent pulping liquor recovery and regeneration operations are inefficient and in this way the white liquor also may contain, besides sodium carbonate, other inert or unregenerated pulping chemicals, generally in the form of sodium- and sulphur-containing compounds. Upon concentration of the white liquor in accordance with the procedure of the above-mentioned Canadian Pat. No. 915,361 and U.S. Pat. No. 3,746,612, some of the unregenerated pulping chemicals are coprecipitated with the sodium chloride. If the separation and recycle procedure of the above-mentioned Canadian Pat. No. 915,362 and U.S. Pat. No. 3,740,307, is used the recycle solution will contain the unregenerated pulping chemicals which rapidly would build up and prevent the separation of adequate quantities of pure sodium chloride.